This invention relates generally to a method for modifying by fluorination articles molded from thermoset elastomers, in order to reduce the static and dynamic friction characteristics and to increase the wear life and abrasion resistance of the elastomers. The invention also relates to elastomeric articles modified by the fluorination method.
An elastomer is generally a polymeric material that possesses an inherent property known as elasticity which allows it to return to its original form when released from a deforming load. An elastomer is capable of a substantial degree of stretching under tension before breaking.
Elastomers are commonly used in a wide range of applications. These applications include, but are not limited to: static and dynamic seals, couplings, rollers, bushings, diaphragms, gears and shock absorbers. The performance of elastomers used in these and other applications is often limited because of the inherent high friction and high wear characteristics of these materials. Several methods of improving these undesirable properties are currently in use in industry. The most common of these methods is to add to the elastomer formulation a material that is typically referred to as a "process aid", or "internal lubricant". Many such materials are commercially available, including waxes, silicones, fatty acids, fluorocarbons, polyethylenes, etc. When the elastomer formulation is processed into a finished part via molding, the process aid diffuses to the surface of the article. During operation, a lubricative layer is formed between the elastomer and mating surface, which lowers friction and, to some extent, increases wear life. This effect, however, is often short-lived since the internal lubricant gradually wears away. This technique introduces foreign material to the elastomer matrix and also has a tendency to induce non-uniform performance since the lubricant's diffusion rate is controlled by temperature and pressure, both of which can vary considerably over the operational conditions
Another common technique which is used to improve the friction characteristics of certain elastomers is to etch the surface of the finished article with a dilute aqueous chlorine solution, derived from muriatic acid and sodium hypochlorite. However, the use of this procedure is generally limited to NBR-based elastomers. In addition, the etched layer is very thin and quickly wears away during normal operation so that the friction problem returns.
Other techniques are also used to reduce friction in elastomeric materials. These include coating finished articles with PTFE, silicone grease or other external lubricants. Occasionally, a low friction coating may be used in tandem with an internal lubricant.
The common factor between all the techniques discussed above is that the benefits that arise from each technique are short-term only. The root of the problem, i.e. the inherent high friction in the elastomeric materials, is not addressed.
It is highly desirable, therefore, to generate low friction elastomeric materials which retain low friction and high wear resistance characteristics for substantial periods of time.